You need to carry at least one dive knife with you every time that you go spearfishing. A knife isn't simply for dispatching a fish but is a life-saving tool if you get wrapped up in your line by a fish that's running circles around you. Having a back up knife is important as it's easy for a fish to knock the knife out of your hand, or to drop it when you're slapped in the face by it's tail. What do you do then?
Dive Newcastle stocks a range of quality spearfishing knives but, like anything, you get what you pay for. A cheaper dive knife will often become blunt and rust quite quickly! It is a good idea to budget $50-$70 for a high quality spearfishing knife that will last you a lifetime. Most of the rust to appear on a dive blade will be superficial but still detracts from the performance of the knife.
A dagger is perfect for spearfishing as it has a pointed tip to spike your fish, a smoothly sharpened blade for slicing fish for burly, and a serrated edge for cutting through ropes. It is also important to consider how the dive knife is strapped and how it can be taken out for use. Many people opt to have their knife on their belt for easy access, others run it around there leg but this can be harder to reach, especially if you're tangled.